Is the theory correct? Two Clues from two Types of Star Clusters  Open Cluster Globular Cluster 

Star Clusters Group of stars formed from fragments of the same collapsing cloud Same age and composition; only mass distinguishes them Two Types: Open clusters (young  birth of stars) Globular clusters (old  death of stars)

What do Open Clusters tell us? Hypothesis: Many stars are being born from a interstellar gas cloud at the same time Evidence: We see “associations” of stars of same age  Open Clusters

Why Do Stars Leave the Main Sequence? Running out of fuel

Stage 8: Hydrogen Shell Burning Cooler core  imbalance between pressure and gravity  core shrinks hydrogen shell generates energy too fast  outer layers heat up  star expands Luminosity increases Duration ~ 100 million years Size ~ several Suns

Stage 9: The Red Giant Stage Luminosity huge (~ 100 Suns) Surface Temperature lower Core Temperature higher Size ~ 70 Suns (orbit of Mercury)

The Helium Flash and Stage 10 The core becomes hot and dense enough to overcome the barrier to fusing helium into carbon Initial explosion followed by steady (but rapid) fusion of helium into carbon Lasts: 50 million years Temperature: 200 million K (core) to 5000 K (surface) Size ~ 10  the Sun Helium is burning in the core. Hydrogen is burning in shell and around core.

Stage 11 Helium burning continues Carbon “ash” at the core forms, and the star becomes a Red Supergiant Duration: 10 thousand years Central Temperature: 250 million K Size > orbit of Mars

Deep Sky Objects: Globular Clusters Classic example: Great Hercules Cluster (M13) Spherical clusters may contain millions of stars Old stars Great tool to study stellar life cycle

Observing Stellar Evolution by studying Globular Cluster HR diagrams Plot stars in globular clusters in Hertzsprung-Russell diagram Different clusters have different age Observe stellar evolution by looking at stars of same age but different mass Deduce age of cluster by noticing which stars have left main sequence already

Catching Stellar Evolution “red-handed” Main-sequence turnoff

Type of Death depends on Mass Light stars like the Sun end up as White Dwarfs Massive stars (more than 8 solar masses) end up as Neutron Stars Very massive stars (more than 25 solar masses) end up as Black Holes

Reason for Death depends on Mass Light stars blow out their outer layers to form a Planetary Nebula The core of a massive star (more than 8 solar masses) collapses, triggering the explosion of a Supernova Also the core of a very massive stars (more than 25 solar masses) collapses, triggering the explosion Supernova

Light Stars: Stage 12 - A Planetary Nebula forms Inner carbon core becomes “dead” – it is out of fuel Some helium and carbon burning continues in outer shells The outer envelope of the star becomes cool and opaque solar radiation pushes it outward from the star A planetary nebula is formed Duration: 100,000 years Central Temperature: 300  106 K Surface Temperature: 100,000 K Size: 0.1  Sun

Deep Sky Objects: Planetary Nebulae Classic Example: Ring nebula in Lyra (M57) Remains of a dead, exploded star We see gas expanding in a sphere In the middle is the dead star, a “White Dwarf”

Stage 13: White Dwarf Core radiates only by stored heat, not by nuclear reactions core continues to cool and contract Size ~ Earth Density: a million times that of Earth – 1 cubic cm has 1000 kg of mass!

Stage 14: Black Dwarf Impossible to see in a telescope About the size of Earth Temperature very low  almost no radiation  black!

More Massive Stars (M > 8MSun) The core contracts until its temperature is high enough to fuse carbon into oxygen Elements consumed in core new elements form while previous elements continue to burn in outer layers

Evolution of More Massive Stars At each stage the temperature increases  reaction gets faster Last stage: fusion of iron does not release energy, it absorbs energy  cools the core  “fire extinguisher”  core collapses Region of instability: Variable Stars

Supernovae – Death of massive Stars As the core collapses, it overshoots and “bounces” A shock wave travels through the star and blows off the outer layers, including the heavy elements – a supernova A million times brighter than a nova!! The actual explosion takes less than a second

Type I vs Type II Supernovae The time dependence of the intensity of the light from a supernovae is called a light curve. Light curves with two characteristic shapes are observed: Type I Supernova (carbon-detonation): Slight delay in the decay of the brightness. Spectrum shows little hydrogen or helium. the progenitor star is a carbon core of an older star Type II Supernova (core-collapse): Spectrum shows more hydrogen and helium (outer layers). Situation as discussed above

Type I – “Carbon Detonation” Implosion of a white dwarf after it accretes a certain amount of matter, reaching about 1.4 solar masses Very predictable; used as a standard candle Estimate distances to galaxies where they occur Chandrasekhar limit

Type II – “Core Collapse” Implosion of a massive star Expect one in our galaxy about every hundred years Six in the last thousand years; none since 1604

Supernova Remnants Crab Nebula (M1) from Supernova in 1054 AD recorded by the Chinese and Native Americans, among others Was visible in daytime (it’s relatively close) Many supernovae in other galaxies have been observed in the 20th century, most notably Supernova 1987 A, in the Large Magellanic Cloud. From Vela Supernova Crab Nebula

SN1987A Lightcurve Occurred in LMC, a small nearby galaxy (knew distance immediately) Progenitor was known previously (a blue supergiant) Neutrino pulse observed ~20 hours before light Bright ring in pic probably the planetary nebula previously blown off, heated by radiation from the blast

What’s Left? Type I supernova Type II supernova Nothing left behind While the parent star is destroyed, a tiny ultra-compressed remnant may remain – a neutron star This happens if the mass of the parent star was above the Chandrasekhar limit

More Massive Stars end up as Neutron Stars The core cools and shrinks Nuclei and electrons are crushed together Protons combine with electrons to form neutrons Ultimately the collapse is halted by neutron pressure, the core is composed of neutrons Size ~ few km Density ~ 1018 kg/m3; 1 cubic cm has a mass of 100 million kg! Manhattan

Formation of the Elements Light elements (hydrogen, helium) formed in Big Bang Heavier elements formed by nuclear fusion in stars and thrown into space by supernovae Condense into new stars and planets Elements heavier than iron form during supernovae explosions Evidence: Theory predicts the observed elemental abundance in the universe very well Spectra of supernovae show the presence of unstable isotopes like Nickel-56 Older globular clusters are deficient in heavy elements GCs contain “first generation” stars – later generations have higher proportion of heavy elements

Neutron Stars (“Pulsars”) First discovered by Jocelyn Bell (1967) Little Green Men?!? Nope… Rapid pulses of radiation Periods: fraction of a second to several seconds Small, rapidly rotating objects Can’t be white dwarfs; must be neutron stars Hewish (Bell’s advisor): explanation, 1974 Nobel Prize Can’t be white dwarfs because they would fly apart – too big!

The “Lighthouse Effect” Pulsars rotate very rapidly Extremely strong magnetic fields guide the radiation Results in “beams” of radiation, like in lighthouse Spinning so fast b/c of angular momentum, like an ice skater We can see them if the “beam” sweeps across the earth Width of beam may only be a few degrees Other wavelengths also produced, but radio typically the strongest

Super-Massive Stars end up as Black Holes If the mass of the star is sufficiently large (M > 25 MSun), even the neutron pressure cannot halt the collapse – in fact, no known force can stop it! The star collapses to a very small size, with ultra-high density Nearby gravity becomes so strong that nothing – not even light – can escape! The edge of the region from which nothing can escape is called the event horizon Radius of the event horizon called the Schwarzschild Radius

How might we “see” them? Radiation of infalling matter Gravitational effects, if evidence that an unseen partner is both very massive and very small

Evidence for the existence of Black Holes Fast Rotation of the Galactic Center only explainable by Black Hole Other possible Black Hole Candidates: Cygnus X-1 (X-ray source), LMC X-3 Observational evidence very strong

Black Holes – The Center of Galaxies? IR picture of the center of the Milky Way

Novae – “New Stars” Actually an old star – a white dwarf – that suddenly flares up Accreted hydrogen begins fusing Usually lasts for a few months May repeat (“recurrent novae”)

Review: The life of Stars